What are trimethoprim and sulfamethoxazole? How do they interact with other drugs?

Folate

Effect: Supplementation Likely Helpful

Both trimethoprim and sulfamethoxazole interfere with folate: The sulfamethoxazole makes it hard for invading bacteria to manufacture folate, and the trimethoprim makes it hard for bacteria to use the folate. The net effect is to starve the bacteria of this necessary vitamin.

Humans and other mammals are much less affected by these antibiotics than are bacteria, because of the different way humans process folate. However, trimethoprim can still interfere to some extent with the body’s ability to utilize this essential nutrient. Folate supplementation may be helpful if one takes this antibiotic for a long period of time (to prevent urinary tract infections, for example).

PABA (Para-Aminobenzoic Acid)

Effect: Interference with Action of Drug

The supplement PABA may make trimethoprim/sulfamethoxazole less effective. Persons being treated with this drug should not take PABA except on medical advice.

Potassium

Effect: Possible Harmful Interaction

Trimethoprim/sulfamethoxazole might increase levels of potassium in the body. Therefore, persons on long-term treatment with this antibiotic should not take potassium supplements except on the advice of a physician.

White Willow

Effect: Possible Negative Interaction

The herb white willow contains substances very similar to aspirin. On this basis, one should not combine white willow with trimethoprim or sulfamethoxazole.

St. John’s Wort and Other Herbs

Effect: Potential Increased Risk of Photosensitivity

Sulfa drugs can cause increased sensitivity to the sun. Various herbs, including St. John’s wort and dong quai, can also cause this problem. Combined treatment with herb and drug might increase the risk further.

Bibliography

Alappan, R., M. A. Perazella, and G. K. Buller. “Hyperkalemia in Hospitalized Patients Treated with Trimethoprim-Sulfamethoxazole.” Annals of Internal Medicine 124 (1996): 316-320.

Vinnicombe, H. G., and J. P. Derrick. “Dihydropteroate Synthase from Streptococcus pneumoniae: Characterization of Substrate Binding Order and Sulfonamide Inhibition.” Biochemical and Biophysical Research Communications 258 (1999): 752-757.